U.S. Pat. No. 3,046,801 (Pravel) describes such a friction driving mechanism where each ball may rotate about an oblique axis through the ball. A disadvantage of this construction is that the force which may be transmitted by the balls to the axially movable shaft, is not sufficient high even if one has tried to remedy this by pressing the balls against the shaft by means of special rings. It is also assumed that the lifetime is insufficient due to wear.
U.S. Pat. No. 4,203,328 and U.S. patent Re 27,711 describes a friction driving mechanism which is able to convert a rotating movement into an axial movement. Also these constructions seem to have a limited lifetime due to wear.
U.S. Pat. No. 3,443,443 describes a friction driving mechanism for converting a rotating movement into an axial movement. This mechanism consists of driving means and friction means connected to a member rotating with its rotating member, the shaft. The friction means comprises a holder embedding a number of mutually movable and with the holder in relation to the shaft rotatable balls, the axis of rotation of said balls being oblique in relation to the longitudinal axis of the shaft. A disadvantage of this mechanism is that the balls in the friction means primarily are worn along equator with regard to their axes of rotation.